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Oil between the piston and the casing manage to move with the piston?

  1. Aug 5, 2005 #1
    How does the oil between the piston and the casing manage to move with the piston? Is their no leak of the oil?

    And does the piston ring actually rub against the walls of the casing? Doesn't it generate too much friction?

    Does anyone know a good site where they show how the dimensions of piston, piston ring, etc. are calculated?
  2. jcsd
  3. Aug 5, 2005 #2
    To answer you question about how the oil moves or doesn't move with the piston action, I'd like to suggest the study of fluid mechanics. Often when fluid mechanics are studied we are concerned with the flow of some fluid and how it interacts with it surrounding both in terms of momentum and energy. Upon, stodying these relations we find that fluid mechanics is highly based on differential equations. One aspect of differential equations is that there are infinite solutions to a given problem until initial conditions are defined. More specifically we can let the boundary conditions equal the initial conditions. In you example, this means the fluid in contact with the piston move with the same velocity as the piston, and that the fluid in contact with the outer cylindrical walls are stagnant (or stationary). This is assumed we have a viscouse fluid and that there is a "no slip" condition. The two metal pieces (piston and walls) do not contact. There is a thing layer of oil between the two. However, it should be noted as the temperature increases the two boundaries become closer and the pressure on the fluid increases. If we wanted to simplistically model the velocity profile of the oil, we find that the velocity of the fluid is proportional to the derivative of the "stress" on the fluid. In more basic terms this means that the velocity profile of the fluid going from the piston to the wall is more like Ae^(-c/(R-r)). Where A is the velocity of the piston and c is a function of the fluid viscosity. r is oriented ina radial direction outwards from the piston to the wall. R is that total distance between the surfaces of the pistaon and the inner wall.
  4. Aug 5, 2005 #3


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    You are one scary dude. I couldn't even read most of that, but it sure looks impressive. :biggrin:
    Sid, if you're referring to a standard automotive engine, the oil rings are actually a low-tension honeycomb setup with 'cells' designed to trap oil and keep it in contact with the cylinder walls. The compression rings scrape the walls clean, as well as protect the oil rings and other 'below decks' stuff from the heat, pressure, and pollutants of the combustion process.
  5. Aug 5, 2005 #4


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    If you'll notice on the walls of a cylinder, they are "crosshatched." This also helps retain oil on the walls. Yes, this means there is oil in the combustion chamber during ingnition and that every IC engine burns some oil.

    Not sure why I mentioned that....oh well.
  6. Aug 6, 2005 #5


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    Perhaps because it's relevant? On the other hand, though, when we were doing engine building in the old days, we made sure that the cylinders were honed to as close to a mirror finish as could be achieved. Every reduction in friction is an increase in available rear-wheel horsepower. That's why oil rings are designed as they are; the low tension minimizes friction with the cylinder walls, while the 'cells' ensure that those walls are lubricated.
  7. Aug 6, 2005 #6
    The cross-hatched pattern is made using a cylinder hone and their purpose is to help seat the piston rings to the cylinder wall as the engine breaks in.
  8. Aug 6, 2005 #7


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    Ahhh... not familiar with standard rings, so I hadn't thought of that. I used race-only pistons with 1/16th rings. (The rings are supposed to be good for about 10 passes in the quarter before needing to be changed. I put 30,000 miles on the first set in my Roadrunner and then only replaced them because we had to pull it apart to change the cam.) The break-in period was 50 miles tops, without cross-hatching. (Of course, we also mounted the pistons backward for that little extra something. :biggrin: )
  9. Aug 6, 2005 #8
    But the seating process only takes a few minutes to a few tens of hours depending on the ring composition and the ring facing material. The cross hatch remains after the seating process though not as pronounced as a newly built engine.

    Many piston/cylinder devices utilize an oil control ring. This ring is most commonly an upper thin ring, and spring like expander and then a lower thin ring---so its actually three piece but it's called an oil ring. This combination of ring-expander-ring allows, in conjunction with an oil port(s), for proper oil control which limits the amount of oil injected into the combustion process, cools the pison skirt, and lubricates the skirt/cylinder interface. The top two rings rub against the wall with limited lubrication(older engines will have a taper and usually a lip at the top of the cylinder where the top ring has worn away the cylinder) but the composition of the ring ensures it will last the desired time span. For production engines or mildly modified non-race you will get a ductile iron ring with a chrome-moly face. The chrome-moly face greatly increases the usable lifespan of the ring. Pure race rings are ductile iron only and they only last about one race.

    There are dozens of ring compositions for different apps but this goes above and beyond your question. The answer is the cross hatch retains a thin oil film while an oil control ring and oil port(s) constantly lubricate the piston's skirt and cylinder wall.
  10. Aug 6, 2005 #9


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    That's what I was referring to. The expander is what I consider to be a 'cell' structure.

    You must be talking about roundy-round races. I did my rebuild in '75, and my rings are double-moly intended for quarter-mile and tractor pulling competitions. (The pistons are 12:1 forged aluminum TRW's, with the wrist-pin offset reversed.) I run an .008 end gap, as opposed to the recommended .035. Unfortunately, I can't afford to gas up at the airport. :grumpy:
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